Pieces around 12" can be an issue.
Start with the fence and see if aligns parallel to the miter slot when locked down. Can you wiggle it at all with firm pressure? You have to fix this first.
Use a clean and sharp blade. Dirty blades with gum in the gullets or on the plates don't cut accurately.
Put the splitter back on the saw, IF it's not on already. The splitter does not allow the work to twist off the fence at the rear after you've made a kerf. A feather board won't help much, if at all, with a short piece and may get in the way.
You need a push block, not a push stick, that will allow you to press down and in toward the blade simultaneously. There are a few commercial versions, but you can make one easily.
The edge you register against the fence must be straight .... no curves or bumps. Raise the blade to about 1/4" to 1/2" above the thickness. The higher the blade the better it will rip, since the tips and cutting in a more downward direction as opposed to into the work, then downward. Some don't like a fully exposed blade, but other will say if you get cut by either the difference will be minimal.
You must push the work all the way through the back of the blade and DO NOT grab it, let it either slide on to your outfeed table (a safety must) or fall on the shop floor. You can use more workpieces to push each other through, but you must them add downward pressure with a push block. There are roller attachments for your fence that press down in front and at the rear of the blade, BUT I find them cumbersome and always in the way. See them attached to the Biesemeyer fence. Also notice a full height splitter behind the blade: